1. Field of the Invention
The present invention relates to rubber injection molding devices and rubber product manufacturing methods using injection molding.
2. Description of the Related Art
Conventionally, the injection molding method has been used broadly as a rubber product molding method. FIG. 8 shows an example of that which has been used conventionally as a rubber injection molding device. In this figure, 200 denotes an injection mold that has a molding cavity 202. 204 denotes an injecting machine, and 206 denotes a feed machine. The injecting machine 204 has an injection chamber 210 in an injection cylinder 208, where the rubber, which is charged to a set charge amount, in this injection chamber 210 and is injected in a single stroke into the molding cavity 202 of the mold 200 from a nozzle 214 at the tip of the injection cylinder 208 by the motion in the downward direction (in the figure) of an injection plunger 212.
The injection cylinder 208 is equipped with heating apparatus 216 to heat the rubber within the injection chamber 210. Moreover, in the injection cylinder 208, a straight injection duct 218 is formed from the injection chamber 210 to the nozzle 214.
On the other hand, the feed machine 206 has a feed cylinder 220 and a screw 222, equipped therein, where rubber, supplied from a supply aperture 224, is mixed by the rotation of the screw 222, and after enhancing a molten state, the rubber is fed into the injection chamber 210 from a feed aperture 226 at the tip of the feed cylinder 220 through a feed duct 228 that connects together the feed aperture 226 and the injection chamber 210 of the injection cylinder 208.
The injection plunger 212 moves backwards (in the upwards direction in the figure) due to the pressure as the rubber is fed, or in other words, charged, into the injection chamber 210. When the amount of rubber charged into this injection chamber 210 reaches set charge amount that is appropriate for a single-stroke injection, the feeding of the rubber by the feed machine 206 is stopped. In other words, the injection chamber 210 and the injection plunger 212 have the function of measuring the rubber that is charged therein.
The heating apparatus 232 is provided for heating the rubber within the cylinder in the feed cylinder 220 in the feed machine 206 as well. A one-way valve (a reverse flow prevention valve) 230 for preventing the reverse flow of rubber within the injection chamber 210 into the feed cylinder 220 is equipped in the feed duct 228, described above, that connects between the feed machine 206 and the injecting machine 204. In FIG. 8, 234 denotes a drive device for the injection plunger 212, and 236 denotes a drive device for the screw 222 in the feed machine 206.
FIGS. 9A and 9B show the operation of the rubber injection molding device. As illustrated, the rubber injection molding device operates such that feeding of the rubber from the feed machine 206 charges the rubber into the injection chamber 210 of the injection cylinder 208, where, at the same time, the injection plunger 212 moves backwards (in the upwards direction in FIG. 9A). When the injection plunger 212 arrives at the back limit, established in advance, the feeding of the rubber from the feed machine 206 is stopped. The back limit of the injection plunger 212 is set in advance to a position wherein the amount of rubber charged into the injection chamber 210 is a set charge amount appropriate for a single-stroke injection.
When the set amount of rubber has been charged into the injection chamber 210 in this way, then, as shown in FIG. 9B, the injection plunger 212 moves forward (in the downward direction in the figure), causing the rubber that is charged into the injection chamber 210 to pass through the straight injection duct 218 to be injected, in a single stroke, from the nozzle 214 into the molding cavity 202 of the mold 200. There is a certain amount of rubber remaining in the straight injection duct 218 when the advancement of the injection plunger 212 has finished injecting the rubber from within the injection chamber 210 into the molding cavity 202. This remaining rubber will be injected into the molding cavity 202 in the next injection cycle.
The rubber injected into the molding cavity 202 is cured for a specific amount of time within the mold 200, which has been brought to the vulcanization temperature in advance, after which the rubber is removed from the mold 200. In cure molding of rubber products using this rubber injection mold device, the rubber is injected into the molding cavity 202 at a temperature that is less than the temperature of the mold 200. The injected rubber is heated to the temperature of the mold 200 through being heated by the mold 200, after which vulcanization is performed through maintaining the temperature of the mold at the vulcanization set temperature for a specific amount of time.
For example, conventionally if the rubber temperature within the injection chamber is about 90° C., the temperature of the rubber in the stage wherein it is passing through the nozzle 214 is about 110° C., and the temperature of the rubber at the point in time wherein it has been injected into the molding cavity 202 is 130° C., the rubber that has been injected into the molding cavity 202 is cured through heating for about six minutes with the vulcanization set temperature for the mold set at 170° C.
Although the vulcanization starts gradually for the rubber that has been injected into the molding cavity 202, due to the heating of the mold, when the set temperature of the mold is increased in order to shorten the vulcanization time, there will be over vulcanization near the surface of the product before the temperature rises in the center of the product, and thus in order to cure the produce uniformly five to six minutes of time has been required regardless. (Note that if the type of rubber product is changed, then, of course, the vulcanization temperature, the vulcanization conditions, the time to ramp the rubber up to the vulcanization temperature, and the like, will, of course, be different.) As described above, a certain amount of time is required to heat all of the rubber that has been injected into the molding cavity 202 up to the vulcanization temperature in order to perform vulcanization uniformly when cure molding a rubber product and this [time required for ramping up] is included in the time required for vulcanization.
Conventionally, to make the vulcanization time short in injection molding of rubber products using rubber injection molding devices has been a major issue. The longer the vulcanization time, the less the manufacturing efficiency, which not only requires a greater number of molds, but also increases the amount of space occupied by the vulcanization equipment commensurately.
In order to reduce the vulcanization time when performing cure molding of rubber products using a rubber injection molding device, it is necessary to increase the temperature of the rubber injected into the molding cavity 202. As a means to do so, one can consider increasing at least the set temperature for the rubber within the injection chamber 210.
However, in conventional rubber injection molding devices, when, as shown in FIG. 9B, the rubber within the injection chamber 210 is injected by the forward motion of the injection plunger 212, there will be residual rubber within the straight injection duct 218 and within the feed duct 228, which connects between the feed aperture 226 at the tip of the feed cylinder 220 and the injection chamber 210, and thus if the set temperature for the injection chamber 210 is increased too far, then scorching (burning of the rubber) will result. Because scorched rubber has an adverse effect on product quality, the problem cannot be solved by simply increasing the set temperature.
When it comes to the residual rubber in the injection duct 218, not only can the injection duct 218 (formed in the nozzle 214) be shortened by having the tip part of the injection chamber 210 be the same shape as the tip part of the injection plunger 212 as shown in, for example, JP-A-2003-11189, JP-B-3174346, and EP0 287 001, but also this residual rubber can be discharged together with the runner on the mold side by having a shape that widens towards the bottom.
However, in these conventional devices, no consideration has been given to the rubber that remains in the feed duct 228. Even if the temperature setting for the heating apparatus 232 of the feed machine is increased in order to increase the temperature of the rubber that is supplied to the injection chamber 210, the temperature of the rubber that remains within the feed duct 228 will decline until the start of the next feed cycle by the action of the injection plunger 212, which will cause variability in the temperature distribution within the injection chamber 210. If, in order to prevent this reduction in temperature, heating apparatus are provided around the periphery of the feed duct, then scorching will become a problem. Conventionally, these problems have not been addressed, nor have any means of resolution thereof been disclosed or proposed.